Polarized EXAFS and XANES studies on single crystals of several heme enzymes and model compounds will be-performed. The heme en- zymes include resting and camphor-bound cytochrome p-450 from Ps. putida, and the resting form of yeast cytochrome c peroxidase as well as its cyanide adduct and enzyme-substrate complex. These heme enzymes catalyze a diverse series of rections. Cytochrome c peroxidase catalyzes the breakdown of organic peroxides and ex- hibits catalytic intermediates which are similar in structure and function to catalase and horse radish peroxidase. The cytochromes P-450 catalyze a mixed function bond-breaking of hydrocarbons by cleaving molecular oxygen. This class of enzymes is characterized by a diverse selectivity of substrates and cytochromes P-450 are found in regulatory, oncogenic, and toxological systems. Both the peroxidases and cytochrome P-450 share stereochemical aspects of their catalytic mechanisms and they are similar in certain respects to mitochondrial cytochrome oxidase. A polarized EXAFS study of these enzymes will complement x-ray crystalloqrahic and solution EXAFS studies by providing highly accurate heme in-plane and axial geometries of the catalytic intermediates which are stable in the crystal. ln addition, the measurement of polarized XANES spectra on these enzymes and models coupled with X-Alpha Scattered Wave molecular orbital calculations will provide a description of the electronic basis for catalysis by these enzymes. Finally, the polarized EXAFS and XANES experiments on the model compounds and enzymes will provide a detailed stereochemical decription of the heme second and third nearest neiqhbors.